Chris Smylie scored twice as Otago shocked Canterbury 37-22 at Christchurch's Jade Stadium to progress to the NPC final.
The Otago scrum-half led the scoring with two tries as his province came out on top in a surprisingly open display of rugby for such a crucial derby fixture - and their second win over the Red and Blacks in a fortnight.
Having scored a 24-19 win over their neighbours in the penultimate week of the round-robin, they were even more emphatic this time around.
Captain Craig Newby, fullback Craig Clare and flanker Josh Blackie scored the other tries, with All Black Nick Evans kicking two penalties and three conversions.
The champions led briefly, after tries from Ben Blair and Richie McCaw overturned a 10-3 Otago half-time advantage.
But tries by Clare, Smylie and Blackie in the final quarter-hour settled the issue.
Newby said after the game: "I don't know whether I'm real tired or just amazed by our performance out there.
"Though we beat them two weeks ago we knew they'd be even tougher tonight.
"But it was an unbelievable performance from our guys. I mean five tries - you wouldn't dream it would happen at Jade Stadium."
Canterbury captain Reuben Thorne added: "They were really hungrier than us, especially in the first half because we couldn't get anything going. In the second half they just forced us into a lot of errors and capitalised on them.
"We were a yard off their game and they put us under a lot of pressure and those mistakes cost us. We're very disappointed."
The game marked Canterbury coach Aussie McLean's swansong, as he defects to Wellington's Hurricanes for the 2006 Super 14 season.